/Engine/Source/Editor/LevelEditor/Public/LevelEditor.h
they must be added before the LevelEditor is init
virtual void AddCustomFilterToOutliner ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [FFilterBase](API\Developer\ToolWidgets\Filters\FFilterBase)< const [ISceneOutlinerTreeItem](API\Editor\SceneOutliner\ISceneOutlinerTreeItem) & >> InCustomFilter )